Fiber to the Antenna (FTTA) Revolutionizes Cell Site Builds

Cellular carriers have a big challenge. Their customers expect fast Internet connections between the network and their smartphones and tablets from wherever they happen to be. This demand for high-speed service is taxing network capacity, severely leaving the carriers scrambling to expand network coverage and capacity just to keep up.

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The carriers have several options.

First, they can add new cell sites. This approach will improve the area coverage and capacity performance by providing greater data-handling capacity to each wireless device. Putting up towers to support large macrocells is difficult; especially in dense urban areas where a majority of customers reside and work. While there is a new focus on small cells and distributed antenna systems (DAS) for improving coverage and capacity both in outdoor networks and inside buildings, that aspect still is a minor part of network builds though progressing steadily.

The other option is to add or upgrade existing sites with next-generation base stations, and that activity already is well underway. Cellular carriers are deploying state-of-the-art 4G/LTE technology that has built-in features and techniques for greater data-handling capacity at faster download speeds than conventional 3G networks provide today. To achieve that high-speed performance, the 4G/LTE radio equipment manufacturers have incorporated advanced modulation and transmission techniques along with a radical new split-mount radio configuration that enhances RF transmission performance. Earlier-generation 2G and 3G cellular radios are contained in a single unit housed in a shelter or cabinet at the base of the tower. These modular units exchange baseband signals with the carrier's mobile switching center on the network side, and send and receive RF signals to and from the antennas over coaxial cable runs on the tower side. Long coaxial cables, unfortunately, significantly reduce the RF power between the radio and the antenna.

By contrast, new 4G/LTE radio split-mount configurations include the Baseband Unit (BBU) that is installed in a shelter or cabinet at the base of a tower, and Remote Radio Units (RRUs) that are installed up on the tower next to the sector panel antennas. One BBU can support multiple RRUs. A vertical fiber optic cable run connects the BBU to the RRUs. With fiber optic cable, signal losses between the BBU and RRUs are eliminated and RF signal strength delivered to the antenna from the RRU is much higher. A short coaxial cable jumper is used to connect the RUU to the antenna. Despite the performance advantages, operating RRUs high up on the tower create other challenges for the carriers. Installing fiber optic cables on towers requires careful handling, and the RRUs must be powered since they are active electronic devices.

Again, the carriers have options.

The simplest arrangement is referred to as a "home run" wherein separate pre-connectorized fiber optic cables and separate copper power cables connect to each RRU. Home run cables must be sized and cut to the appropriate length for each RRU. The downside is that there are a lot of cables needed to connect to all the RRUs, especially in a high-density application. Moreover, there is no surge protection provided at the top of the tower for the power cables.

The second option, referred to as "hybrid cables," is widely used [Figure 1]. Hybrid cables house within a single weatherproof sheath, a combination of multiple copper wire pairs for powering the RRUs, and multiple fiber strands for signaling between the BBU and RRUs. At the top of the hybrid cable vertical run, the appropriate number of power wires and fiber strands are "broken out" into separate cables that plug into the individual RRUs. The hybrid cable simplifies the installation by reducing the number of vertical cable runs needed to serve the total complement of RRUs. However, an amount of customization is required because the breakout lengths and the type of connectors must be customized for each RRU. And as with home runs, there is no surge protection at the top of the vertical run. On very tall towers where the RRUs may be mounted above 300 feet, special hybrid cables with large-gauge copper wires are needed to deliver power to the RRUs at such heights. These are very heavy cables that require extra installation effort.

A third option is to install separate power cables and fiber cables that are each terminated in a junction box located at the height of the RRUs [Figure 2]. With this "terminated cable" arrangement, technicians only install power and fiber cable jumpers from the respective junction boxes to the RRUs. Surge protection is optional in the power cable junction box. The advantage of the terminated cable approach is that fewer vertical cables are needed to serve both the current and planned RRU deployments. Installation is simplified because the feeder cables are generally sized for maximum capacity. For instance, a single vertical fiber cable can be configured with 48 strands that terminate on a junction box to serve as many as 12 four-fiber RRUs. Similarly, a single power cable feeder cable with typically 6 or 8 AWG wires can handle from four to six RRUs. With the vertical cables in place, technicians only need to install custom jumpers between the junction boxes and the RRUs as new RRUs are added. A second or third power cable can be installed when the number of RRUs grows to maximum configuration. The terminated cable approach simplifies RRU deployment with fewer vertical feeder cable runs.

Every tower site is different and tower lease contracts fluctuate with the type and number of RRUs and associated cables needed in different configurations. TESSCO can provide all fiber-to-the-antenna (FTTA) solutions, depending on the customer's requirements, along with the site materials, and installation tools and supplies to help the carriers meet their coverage and capacity challenges.

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